Semiconductor device and driving method thereof

ABSTRACT

A semiconductor device includes a photodiode, a first transistor, a second transistor, and a third transistor. The second transistor and the third transistor have a function of retaining a charge accumulated in a gate of the first transistor. In a period during which the second transistor and the third transistor are off, a voltage level of a voltage applied to a gate of the second transistor is set to be lower than a voltage level of a source of the second transistor and a voltage level of a drain of the second transistor, and a voltage level of a voltage applied to a gate of the third transistor is set to be lower than a voltage level of a source of the third transistor and a voltage level of a drain of the third transistor.

TECHNICAL FIELD

The technical field relates to a photosensor and a driving methodthereof. The technical field also relates to a display device includinga photosensor and a driving method thereof. Further, the technical fieldrelates to a semiconductor device including a photosensor and a drivingmethod thereof.

BACKGROUND ART

In recent years, a display device provided with a light-detecting sensor(also referred to as a “photosensor”) has attracted attention. In thedisplay device including a photosensor, a display screen also serves asan input region. A display device having an image capturing function isan example of such a display device (see Patent Document 1, forexample).

Examples of a semiconductor device provided with a photosensor are a CCDimage sensor and a CMOS image sensor. Such image sensors are used in,for example, electronic apparatuses like digital still cameras orcellular phones.

In a display device provided with a photosensor, first, light is emittedfrom the display device. When the light enters a region where an objectto be detected exists, the light is blocked by the object to bedetected, and is partly reflected. The light reflected by the object tobe detected is detected by the photosensor provided in a pixel in thedisplay device, whereby the object to be detected can be found in theregion.

In a semiconductor device including a photosensor, light emitted from anobject to be detected or external light reflected by the object to bedetected is detected directly by the photosensor or condensed by anoptical lens or the like and then detected.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2001-292276

DISCLOSURE OF INVENTION

In a semiconductor device including photosensors, each pixel is providedwith a circuit including a transistor so as to collect electric signalsgenerated by detection of light with the photosensors provided in therespective pixels.

However, it is difficult to accurately convert incident light into anelectric signal because of variation in the electrical characteristicssuch as threshold voltage of a transistor provided in each pixel.

An object of one embodiment of the present invention is to provide asemiconductor device including a photosensor in which incident light canbe accurately converted into an electric signal.

One embodiment of the present invention is a semiconductor deviceincluding a photodiode, a first transistor, a second transistor, a thirdtransistor, and a fourth transistor. The photodiode has a function ofsupplying a charge corresponding to incident light to a gate of thefirst transistor through the second transistor. The first transistor hasa function of accumulating the charge supplied to the gate and afunction of converting the accumulated charge into an output signal. Thesecond transistor has a function of retaining the charge accumulated inthe gate of the first transistor. The third transistor has a function ofdischarging the charge accumulated in the gate of the first transistorand a function of retaining the charge accumulated in the gate of thefirst transistor. The fourth transistor has a function of controllingreading of the output signal. In a period during which the secondtransistor and the third transistor are off, a voltage level of avoltage applied to a gate of the second transistor is lower than avoltage level of a source of the second transistor and a voltage levelof a drain of the second transistor, and a voltage level of a voltageapplied to a gate of the third transistor is lower than a voltage levelof a source of the third transistor and a voltage level of a drain ofthe third transistor.

Another embodiment of the present invention is a semiconductor deviceincluding a photodiode, a first transistor, a second transistor, a thirdtransistor, and a fourth transistor. The photodiode has a function ofsupplying a charge corresponding to incident light to a gate of thefirst transistor through the second transistor. The first transistor hasa function of accumulating the charge supplied to the gate and afunction of converting the accumulated charge into an output signal. Thesecond transistor has a function of retaining the charge accumulated inthe gate of the first transistor. The third transistor has a function ofdischarging the charge accumulated in the gate of the first transistor.The fourth transistor has a function of controlling reading of theoutput signal. In a period during which the second transistor and thethird transistor are off, a voltage level of a voltage applied to a gateof the second transistor is lower than a voltage level of a wiringelectrically connected to the photodiode, and a voltage level of avoltage applied to a gate of the third transistor is lower than avoltage level of a photosensor reference signal line.

Another embodiment of the present invention is a semiconductor deviceincluding a photodiode, a first transistor, a second transistor, a thirdtransistor, and a fourth transistor. The photodiode has a function ofsupplying a charge corresponding to incident light to a gate of thefirst transistor through the second transistor. The first transistor hasa function of accumulating the charge supplied to the gate and afunction of converting the accumulated charge into an output signal. Thesecond transistor has a function of retaining the charge accumulated inthe gate of the first transistor. The third transistor has a function ofdischarging the charge accumulated in the gate of the first transistorand a function of retaining the charge accumulated in the gate of thefirst transistor. The fourth transistor has a function of controllingreading of the output signal. A semiconductor layer of the secondtransistor and a semiconductor layer of the third transistor which areelectrically connected to the gate of the first transistor include anoxide semiconductor. In a period during which the second transistor andthe third transistor are off, a voltage level of a voltage applied to agate of the second transistor is lower than a voltage level on a lowvoltage side of a source and a drain of the second transistor, and avoltage level of a voltage applied to a gate of the third transistor islower than a voltage level on a low voltage side of a source and a drainof the third transistor.

Another embodiment of the present invention is a semiconductor deviceincluding a photodiode, a first transistor, a second transistor, a thirdtransistor, and a fourth transistor. The photodiode has a function ofsupplying a charge corresponding to incident light to a gate of thefirst transistor through the second transistor. The first transistor hasa function of accumulating the charge supplied to the gate and afunction of converting the accumulated charge into an output signal. Thesecond transistor has a function of retaining the charge accumulated inthe gate of the first transistor. The third transistor has a function ofdischarging the charge accumulated in the gate of the first transistor.The fourth transistor has a function of controlling reading of theoutput signal. A semiconductor layer of the second transistor and asemiconductor layer of the third transistor which are electricallyconnected to the gate of the first transistor include an oxidesemiconductor. In a period during which the second transistor and thethird transistor are off, a voltage level of a voltage applied to a gateof the second transistor is lower than a voltage level of a wiringelectrically connected to the photodiode, and a voltage level of avoltage applied to a gate of the third transistor is lower than avoltage level of a photosensor reference signal line.

Note that the semiconductor device refers to an element having asemiconductor property, and all the objects having the element. Forexample, a display device having a transistor is simply referred to as asemiconductor device in some cases.

A semiconductor device including a photosensor in which incident lightcan be accurately converted into an electric signal can be provided.

In addition, since accumulation operation is simultaneously performed inthe plurality of photosensors, the accumulation operation can becompleted in a short time, so that an image of an object to be detectedcan be taken with little blur even when the object moves fast.

Furthermore, a transistor controlling the accumulation operationincludes an oxide semiconductor and thus has an extremely lowoff-current. Consequently, incident light can be accurately convertedinto an electric signal even when the number of photosensors increasesand selection operation requires longer time. Thus, an image with highresolution can be taken.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates an example of a display device according to oneembodiment of the present invention;

FIG. 2 illustrates an example of a display device according to oneembodiment of the present invention;

FIG. 3 is a timing chart according to one embodiment of the presentinvention;

FIG. 4 is a timing chart according to one embodiment of the presentinvention;

FIG. 5 is a timing chart according to one embodiment of the presentinvention;

FIGS. 6A to 6C are circuit diagrams each illustrating an example of aphotosensor according to one embodiment of the present invention;

FIG. 7 illustrates an example of a semiconductor device according to oneembodiment of the present invention;

FIG. 8 is a graph showing the electrical characteristics of atransistor;

FIG. 9 illustrates an example of a semiconductor device according to oneembodiment of the present invention; and

FIG. 10 is a timing chart according to one embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described in detail below with reference todrawings. Note that the following embodiments can be implemented in manydifferent modes, and it is apparent to those skilled in the art thatmodes and details can be modified in various ways without departing fromthe spirit and scope of the present invention. Therefore, the presentinvention is not construed as being limited to the description of theembodiments. Note that in all the drawings for explaining theembodiments, like portions or portions having a similar function aredenoted by like reference numerals, and the description thereof isomitted.

Embodiment 1

In this embodiment, a structure of a display device which is asemiconductor device including a photosensor and operation thereof willbe described with reference to FIG. 1, FIG. 2, and FIG. 3. Note that thedisplay device including a photosensor can be used as an optical touchsensor.

A structure of the display device will be described with reference toFIG. 1. A display panel 100 includes a pixel circuit 101, a displayelement control circuit 102, and a photosensor control circuit 103.

The pixel circuit 101 includes a plurality of pixels 104 arranged inmatrix in a row direction and a column direction. Each of the pixels 104includes a display element 105 and a photosensor 106. The photosensor isnot necessarily provided in each of the pixels 104, and may be providedin every two or more pixels 104. Alternatively, the photosensor may beprovided outside the pixels 104.

A circuit diagram of the pixel 104 will be described with reference toFIG. 2. The pixel 104 includes the display element 105 provided with atransistor 201 (also referred to as a pixel transistor), a storagecapacitor 202, and a liquid crystal element 203; and the photosensor 106provided with a photodiode 204 which is a light-receiving element, atransistor 205 (also referred to as a first transistor), a transistor206 (also referred to as a second transistor), a transistor 207 (alsoreferred to as a third transistor), and a transistor 208 (also referredto as a fourth transistor).

In the display element 105, a gate of the transistor 201 is connected toa gate signal line 209, one of a source and a drain of the transistor201 is connected to a video data signal line 210, and the other of thesource and the drain is connected to one electrode of the storagecapacitor 202 and one electrode of the liquid crystal element 203. Theother electrode of the storage capacitor 202 and the other electrode ofthe liquid crystal element 203 are each kept at a constant voltagelevel. The liquid crystal element 203 is an element including a pair ofelectrodes and a liquid crystal layer interposed between the pair ofelectrodes.

Note that when it is explicitly described that “A and B are connected”,the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein.

The transistor 201 has a function of controlling injection or release ofcharges to or from the storage capacitor 202. For example, when ahigh-level voltage is applied to the gate signal line 209, a voltage atthe voltage level of the video data signal line 210 is applied to thestorage capacitor 202 and the liquid crystal element 203. The storagecapacitor 202 has a function of retaining a charge corresponding to avoltage applied to the liquid crystal element 203. The contrast (grayscale) of light passing through the liquid crystal element 203 is madeby utilizing the change in the polarization direction due to voltageapplication to the liquid crystal element 203, whereby image display isrealized. As the light passing through the liquid crystal element 203,light emitted from a light source (a backlight) on the back surface ofthe display device is used.

For the transistor 201, an amorphous semiconductor, a microcrystalsemiconductor, a polycrystalline semiconductor, an oxide semiconductor,a single crystal semiconductor, or the like can be used. In particular,display quality can be increased by using an oxide semiconductor toobtain a transistor with an extremely low off-current.

Although the display element 105 described here includes the liquidcrystal element, the display element 105 may include other elements suchas a light-emitting element. The light-emitting element is an elementwhose luminance is controlled with current or voltage, and specificexamples thereof are a light-emitting diode and an organiclight-emitting diode (OLED). Note that in this embodiment, a structureof an optical touch sensor (also referred to as an optical touch panel)provided with the display element 105 and the photosensor 106 isdescribed; however, a structure in which a display element is eliminatedcan also be employed. In that case, an image sensor in which a pluralityof photosensors is provided can be obtained.

In the photosensor 106, one electrode of the photodiode 204 is connectedto a wiring 211 (also referred to as a ground line), and the otherelectrode thereof is connected to one of a source and a drain of thetransistor 206. One of a source and a drain of the transistor 205 isconnected to a photosensor reference signal line 212, and the other ofthe source and the drain is connected to one of a source and a drain ofthe transistor 208. A gate of the transistor 206 is connected to a gatesignal line 213, and the other of the source and the drain of thetransistor 206 is connected to a gate of the transistor 205 and one of asource and a drain of the transistor 207. A gate of the transistor 207is connected to a photodiode reset signal line 214, and the other of thesource and the drain of the transistor 207 is connected to thephotosensor reference signal line 212. A gate of the transistor 208 isconnected to a gate signal line 215, and the other of the source and thedrain of the transistor 208 is connected to a photosensor output signalline 216.

For the photodiode 204, an amorphous semiconductor, a microcrystalsemiconductor, a polycrystalline semiconductor, an oxide semiconductor,a single crystal semiconductor, or the like can be used. In particular,a single crystal semiconductor (e.g., single crystal silicon) with fewcrystal defects is preferably used so as to improve the proportion of anelectric signal generated from incident light (the quantum efficiency).As the semiconductor material, it is preferable to use siliconsemiconductor such as silicon or silicon germanium, the crystallinity ofwhich can be easily increased.

For the transistor 205, an amorphous semiconductor, a microcrystalsemiconductor, a polycrystalline semiconductor, an oxide semiconductor,a single crystal semiconductor, or the like can be used. In particular,the transistor 205 has a function of accumulating a charge which issupplied from the photodiode 204 through the transistor 206, in a nodeconnected to the gate and converting the accumulated charge into anoutput signal. Therefore, a single crystal semiconductor is preferablyused to obtain a transistor with high mobility. As the semiconductormaterial, it is preferable to use silicon semiconductor such as siliconor silicon germanium, the crystallinity of which can be easilyincreased.

For the transistor 206, an amorphous semiconductor, a microcrystalsemiconductor, a polycrystalline semiconductor, an oxide semiconductor,a single crystal semiconductor, or the like can be used. In particular,the transistor 206 has a function of retaining a charge of the gate ofthe transistor 205 by controlling on/off of the transistor 206.Therefore, the transistor 206 preferably uses an oxide semiconductor tohave an extremely low off-current.

For the transistor 207, an amorphous semiconductor, a microcrystalsemiconductor, a polycrystalline semiconductor, an oxide semiconductor,a single crystal semiconductor, or the like can be used. In particular,the transistor 207 has a function of discharging a charge of the gate ofthe transistor 205 and a function of retaining a charge of the gate ofthe transistor 205, by controlling on/off of the transistor 207.Therefore, the transistor 207 preferably uses an oxide semiconductor tohave an extremely low off-current.

For the transistor 208, an amorphous semiconductor, a microcrystalsemiconductor, a polycrystalline semiconductor, an oxide semiconductor,a single crystal semiconductor, or the like can be used. In particular,a single crystal semiconductor is preferably used for the transistor 208so that the transistor 208 has high mobility and has a function ofsupplying an output signal of the transistor 205 to the photosensoroutput signal line 216. As the semiconductor material, it is preferableto use silicon semiconductor such as silicon or silicon germanium, thecrystallinity of which can be easily increased.

The display element control circuit 102 is a circuit for controlling thedisplay element 105, and includes a display element driver circuit 107from which a signal is input to the display element 105 through a signalline (also referred to as a source signal line) such as the video datasignal line; and a display element driver circuit 108 from which asignal is input to the display element 105 through a scan line (alsoreferred to as a gate signal line). For example, the display elementdriver circuit 108 on the scan line side has a function of selecting adisplay element included in a pixel in a specified row. The displayelement driver circuit 107 on the signal line side has a function ofsupplying a predetermined-level voltage to the display element includedin the pixel in the selected row. Note that in the display element 105connected to the gate signal line to which a high-level voltage isapplied from the display element driver circuit 108 on the scan lineside, the transistor is turned on and supplied with a voltage at thesame level as the voltage applied to the video data signal line from thedisplay element driver circuit 107 on the signal line side.

The photosensor control circuit 103 is a circuit for controlling thephotosensor 106, and includes a photosensor reading circuit 109 on thesignal line side, where the signal line includes such as the photosensoroutput signal line or the photosensor reference signal line; and aphotosensor driver circuit 110 on the scan line side.

The photosensor driver circuit 110 has a function of performing thehereinafter described reset operation, accumulation operation, andselection operation on the photosensor 106 included in a pixel in aspecified row.

The photosensor reading circuit 109 has a function of extracting anoutput signal of the photosensor 106 included in a pixel in a selectedrow. Note that from the photosensor reading circuit 109, an output ofthe photosensor 106, which is an analog signal, is extracted as it is tothe outside of the display panel with the use of an OP amplifier.Alternatively, the output of the photosensor 106 is converted into adigital signal with the use of an A/D converter circuit and thenextracted to the outside of the display panel.

A precharge circuit included in the photosensor reading circuit 109 willbe described with reference to FIG. 2. In FIG. 2, a precharge circuit200 for one column of pixels includes a transistor 217 and a prechargesignal line 218. Note that the photosensor reading circuit 109 caninclude an OP amplifier or an A/D converter circuit connected to asubsequent stage of the precharge circuit 200.

In the precharge circuit 200, before the operation of the photosensor inthe pixel, the voltage level of the photosensor output signal line isset to a reference voltage level. In FIG. 2, the precharge signal line218 is set to an H level (hereinafter, abbreviated to “H”) so that thetransistor 217 is turned on, whereby the voltage level of thephotosensor output signal line 216 can be set to a reference voltagelevel (here, a low voltage level). Note that it is effective to providea storage capacitor for the photosensor output signal line 216 so thatthe voltage level of the photosensor output signal line 216 isstabilized. Note that the reference voltage level can be set to a highvoltage level. In that case, the conductivity type of the transistor 217is made opposite to that of FIG. 2 and the precharge signal line 218 isset to an L level (hereinafter, abbreviated to “L”), whereby the voltagelevel of the photosensor output signal line 216 can be set to areference voltage level.

Note that an H-level voltage and an L-level voltage in this embodimentcorrespond to a voltage which is based on a high power source voltagelevel and a voltage which is based on a low power source voltage level,respectively. In other words, the H-level voltage is a constant voltageof 3 V to 20 V, and the L-level voltage is a constant voltage of 0 V(also referred to as a reference voltage level or a ground voltagelevel).

Next, the operation of the photosensor 106 will be described withreference to the timing chart of FIG. 3. In FIG. 3, a signal 301, asignal 302, and a signal 303 respectively correspond to the voltagelevels of the gate signal line 213, the reset signal line 214, and thegate signal line 215 in FIG. 2. Further, signals 304A to 304C eachcorrespond to the voltage level of the gate of the transistor 205 (thevoltage level of a node 219 in FIG. 2). The signal 304A shows the casewhere the illuminance of light which enters the photodiode 204 is high(hereinafter, high illuminance), the signal 304B shows the case wherethe illuminance of light which enters the photodiode 204 is middle(hereinafter, middle illuminance), and the signal 304C shows the casewhere the illuminance of light which enters the photodiode 204 is low(hereinafter, low illuminance). Further, the signals 305A to 305C eachcorrespond to the voltage level of the photosensor output signal line216, and the signals 305A to 305C show the high illuminance, the middleilluminance, and the low illuminance, respectively.

In a period A, the voltage level of the gate signal line 213 (the signal301) is set to “H”, the voltage level of the reset signal line 214 (thesignal 302) is set to a level lower than 0 V (hereinafter, abbreviatedto “L2”), and the voltage level of the gate signal line 215 (the signal303) is set to “L”. Next, in a period B, the voltage level of the gatesignal line 213 (the signal 301) is set to “H”, the voltage level of thereset signal line 214 (the signal 302) is set to “H”, and the voltagelevel of the gate signal line 215 (the signal 303) is set to “L”. As aresult, the photodiode 204 and the transistor 206 are turned on, and thevoltage level of the node 219 (the signals 304A to 304C) becomes “H”. Atthis time, reverse bias is applied to the photodiode 204. Further, whenthe voltage level of the precharge signal line 218 is set to the Hlevel, the voltage level of the photosensor output signal line 216 (thesignals 305A to 305C) is precharged to “L”. As described above, theperiod A and the period B are a reset operation period.

Note that in this specification, the level lower than 0 V means,specifically, a voltage level which is lower than the voltage level ofthe source of the transistor 206, the voltage level of the drain of thetransistor 206, the voltage level of the source of the transistor 207,and the voltage level of the drain of the transistor 207. In thisembodiment, the voltage level on the low voltage side of the sources andthe drains of the transistors 206 and 207 is 0 V which is the voltagelevel of the ground line, and the voltage level of the gate signal line213 and the voltage level of the reset signal line 214 in apredetermined period may be referred to as the level lower than 0 V. Inother words, the voltage level on the low voltage side of the source andthe drain of the transistor 206 and the voltage level on the low voltageside of the source and the drain of the transistor 207 can berespectively referred to as the voltage level of the wiring 211connected to the photodiode 204 and the voltage level of the photosensorreference signal line 212, based on the circuit configurationillustrated in FIG. 2.

Next, in a period C, the voltage level of the gate signal line 213 (thesignal 301) is set to “H”, the voltage level of the reset signal line214 (the signal 302) is set to “L2”, and the voltage level of the gatesignal line 215 (the signal 303) is set to “L”. As a result, the voltagelevel of the node 219 (the signals 304A to 304C) starts to decrease withcurrent due to light irradiation to the photodiode 204 (hereinafter,referred to as photocurrent). In the photodiode 204, photocurrent isincreased with increase in the amount of incident light; accordingly,the voltage level of the node 219 (the signals 304A to 304C) changes inaccordance with the amount of incident light. Specifically, sincephotocurrent is largely increased in the signal 304A having a largeamount of incident light, the signal 304A which is the voltage level ofthe node 219 is largely decreased in the period C. Further, sincephotocurrent hardly flows in the signal 304C having a small amount ofincident light, the signal 304C which is the voltage level of the node219 hardly changes in the period C. Furthermore, since the amount ofphotocurrent is increased to an amount between the amount of the signal304A and that of the signal 304C in the signal 304B having a middleamount of incident light, the amount of the signal 304B which is thevoltage level of the node 219 is decreased to an amount between thedecreased amount of the signal 304A and the decreased amount of thesignal 304C. In other words, the photodiode 204 has a function ofsupplying a charge corresponding to the incident light to the gate ofthe transistor 205 through the transistor 206. Then, the channelresistance between the source and the drain of the transistor 205changes. As described above, the period C is an accumulation operationperiod.

Next, in a period D, the voltage level of the gate signal line 213 (thesignal 301) is set to “L2”, the voltage level of the reset signal line214 (the signal 302) is set to “L2”, and the voltage level of the gatesignal line 215 (the signal 303) is set to “L”. The signals 304A to 304Cwhich are the voltage level of the node 219 become constant. Here, thevoltage levels of the signals 304A to 304C in the period D aredetermined by the amount of photocurrent of the photodiode 204 in theabove-described accumulation operation period (the period C). That is,the amount of charge accumulated in the node 219 changes in accordancewith the incident light to the photodiode 204. Note that an oxidesemiconductor is used for a semiconductor layer of the transistor 206and a semiconductor layer of the transistor 207 to obtain transistorswith extremely low off-currents, whereby the accumulated charge can bekept constant until the subsequent selection operation is performed.

Next, in a period E, the voltage level of the gate signal line 213 (thesignal 301) is set to “L2”, the voltage level of the reset signal line214 (the signal 302) is set to “L2”, and the voltage level of the gatesignal line 215 (the signal 303) is set to “H”. As a result, thetransistor 208 is turned on and the photosensor reference signal line212 and the photosensor output signal line 216 are brought intoelectrical conduction through the transistors 205 and 208. Then, thevoltage level of the photosensor output signal line 216 (the signals305A to 305C) is increased in accordance with incident light to theabove-described photodiode 204. Note that in a period before the periodE, the voltage level of the precharge signal line 218 is set to “H” sothat the precharge of the photosensor output signal line 216 iscompleted. Here, the rate at which the voltage level of the photosensoroutput signal line 216 (the signals 305A to 305C) is increased dependson the current between the source and the drain of the transistor 205,namely, the amount of incident light to the photodiode 204 during theperiod C which is the accumulation operation period. As described above,the period E is a selection operation period.

Next, in a period F, the voltage level of the gate signal line 213 (thesignal 301) is set to “L2”, the voltage level of the reset signal line214 (the signal 302) is set to “L2”, and the voltage level of the gatesignal line 215 (the signal 303) is set to “L”. As a result, thetransistor 208 is turned off, and the voltage level of the photosensoroutput signal line 216 (the signals 305A to 305C) becomes constant. Theconstant value here is determined in accordance with the amount ofincident light to the photodiode 204. Thus, the amount of incident lightto the photodiode 204 during the accumulation operation can bedetermined by obtaining the voltage level of the photosensor outputsignal line 216. As described above, the period F is a reading operationperiod.

As described above, in the semiconductor device of this embodiment, thevoltage level of the voltage applied to the gate of the transistor 206and the voltage level of the voltage applied to the gate of thetransistor 207 are set to lower than 0 V in the periods D, E, and Fduring which the transistor 206 is off and the periods A, D, E, and Fduring which the transistor 207 is off. In other words, the voltagelevel of the voltage applied between the gate and the source of thetransistor 206 is set to a level of the threshold voltage of thetransistor 206 or lower, and the voltage level of the voltage appliedbetween the gate and the source of the transistor 207 is set to a levelof the threshold voltage of the transistor 207 or lower. Thus, thefunction of retaining the charge retained in the gate of the transistor205 can be improved.

More specifically, the operation of individual photosensors is realizedby repeatedly performing the reset operation, the accumulationoperation, the selection operation, and the reading operation. Asdescribed above, in this embodiment, the voltage level of the voltageapplied to the gate of the transistor 206 and the voltage level of thevoltage applied to the gate of the transistor 207 are lower than 0 V ina period during which the transistors 206 and 207 are off. Therefore,the transistors 206 and 207 can be turned off more reliably, which canimprove the function of retaining the charge retained in the gate of thetransistor 205 at the time of the accumulation operation and the readingoperation. Further, the function of accurately converting the incidentlight into an electric signal in a photosensor can be improved.Furthermore, a semiconductor layer of the transistor 206 and asemiconductor layer of the transistor 207 preferably use an oxidesemiconductor to obtain transistors having extremely low off-currents.With such a structure, the function of more accurately converting theincident light into an electric signal in a photosensor can be improved.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 2

In this embodiment, a method for driving a plurality of photosensorswill be described.

First, a driving method illustrated in the timing chart of FIG. 4 isdescribed. In FIG. 4, a signal 401, a signal 402, and a signal 403 aresignals showing voltage changes of the reset signal lines 214 inphotosensors of the first row, the second row, and the third row,respectively. A signal 404, a signal 405, and a signal 406 are signalsshowing voltage changes of the gate signal lines 213 in the photosensorsof the first row, the second row, and the third row, respectively. Asignal 407, a signal 408, and a signal 409 are signals showing voltagechanges of the gate signal lines 215 in the photosensors of the firstrow, the second row, and the third row, respectively. A period 410 is aperiod required for one imaging. A period 411 is a period during whichthe reset operation is performed in the photosensor of the second row, aperiod 412 is a period during which the accumulation operation isperformed in the photosensor of the second row, and a period 413 is aperiod during which the selection operation is performed in thephotosensor of the second row. By thus sequentially driving thephotosensor of each row, images can be taken.

It is found here that the accumulation operation in the photosensor ofeach row has a time lag. That is, imaging in the photosensor of each rowis not performed simultaneously, leading to blurring of the image taken.In particular, an image of an object to be detected which moves fast iseasily taken to have a distorted shape: if the object to be detectedmoves in a direction from the first row to the third row, an enlargedimage is taken leaving a trail behind it; and if the object to bedetected moves in the opposite direction, a reduced image is taken.

In order to prevent the time lag of the accumulation operation in thephotosensor of each row, it is effective that the photosensor of eachrow is sequentially driven in a shorter cycle. In that case, however,the output signal of the photosensor needs to be obtained at very highspeed with an OP amplifier or an A/D converter circuit, which causes anincrease in power consumption, and makes it very difficult to obtain animage with high resolution, in particular.

Thus, a driving method illustrated in the timing chart of FIG. 5 isproposed. In FIG. 5, a signal 501, a signal 502, and a signal 503 aresignals showing voltage changes of the reset signal lines 214 in thephotosensors of the first row, the second row, and the third row,respectively. A signal 504, a signal 505, and a signal 506 are signalsshowing voltage changes of the gate signal lines 213 in the photosensorsof the first row, the second row, and the third row, respectively. Asignal 507, a signal 508, and a signal 509 are signals showing voltagechanges of the gate signal lines 215 in the photosensors of the firstrow, the second row, and the third row, respectively. A period 510 is aperiod required for one imaging. A period 511 is a period during whichthe reset operation (at the same time in all the rows) is performed inthe photosensor of the second row, a period 512 is a period during whichthe accumulation operation (at the same time in all the rows) isperformed in the photosensor of the second row, and a period 513 is aperiod during which the selection operation is performed in thephotosensor of the second row.

FIG. 5 is different from FIG. 4 in that the reset operation and theaccumulation operation are each performed in the same period in thephotosensors of all the rows, and after the accumulation operation, theselection operation is sequentially performed in each row withoutsynchronization with the accumulation operation. When the accumulationoperation is performed in the same period, imaging in the photosensor ofeach row is performed simultaneously and an image of an object to bedetected can be easily taken with little blur even when the object movesfast. Since the accumulation operation is performed at the same time, adriver circuit can be provided in common for the reset signal line 214of each photosensor. Further, a driver circuit can also be provided incommon for the gate signal line 213 of each photosensor. Such drivercircuits provided in common are effective in reducing the number ofperipheral circuits or reducing power consumption. In addition, theselection operation sequentially performed in each row makes it possibleto lower the operation rate of an OP amplifier or an A/D convertercircuit when the output signal of the photosensor is obtained. At thistime, the total time for the selection operation is preferably longerthan the time for the accumulation operation, which is particularlyeffective in the case of obtaining an image with high resolution.

Note that FIG. 5 illustrates the timing chart of the method forsequentially driving the photosensor of each row; it is also effectiveto sequentially drive the photosensor only in a certain row in order toobtain an image in a specified region. As a result, a desired image canbe obtained while the operation and power consumption of the OPamplifier or the A/D converter circuit are reduced. Further, a methodfor driving the photosensor of every few rows, namely, some of aplurality of photosensors, is also effective. As a result, an image withdesired resolution can be obtained while the operation and powerconsumption of the OP amplifier or the A/D converter circuit arereduced.

In order to realize the above driving method, the voltage level of thegate of the transistor 205 in each photosensor needs to be kept constanteven after the accumulation operation is completed. Thus, the transistor207 preferably uses an oxide semiconductor to have an extremely lowoff-current as described in the above embodiment.

In the above manner, it is possible to provide a low-power consumptiondisplay device or semiconductor device which allows a high-resolutionimage of an object to be detected to be taken with little blur even whenthe object moves fast.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 3

In this embodiment, a modified example of the circuit configuration ofthe photosensor 106 in FIG. 2 will be described.

FIG. 6A illustrates a configuration in which the transistor 207 forcontrolling the reset operation of the photosensor, which is connectedto the gate of the transistor 205 in FIG. 2, is omitted. In theconfiguration of FIG. 6A, when the reset operation of the photosensor isperformed, the charge accumulated in the gate of the transistor 205 maybe discharged by changing the voltage level of the wiring 211.

FIG. 6B illustrates a configuration in which the transistor 205 and thetransistor 208 are connected in a manner opposite to that in thephotosensor 106 in FIG. 2. Specifically, the one of the source and thedrain of the transistor 205 is connected to the photosensor outputsignal line 216, and the one of the source and the drain of thetransistor 208 is connected to the photosensor reference signal line212.

FIG. 6C illustrates a configuration in which the transistor 208 isomitted in the configuration of the photosensor 106 in FIG. 2. Theconfiguration of FIG. 6C is different from those of FIG. 2 and FIGS. 6Aand 6B: when the selection operation and the reading operation of thephotosensor are performed, a change of the signal corresponding to thecharge accumulated in the gate of the transistor 205 may be read bychanging the voltage level of the photosensor reference signal line 212.

A timing chart relating to the operation of the photosensor 106illustrated in FIG. 6C is shown in FIG. 10. In FIG. 10, a signal 601, asignal 602, and a signal 603 respectively correspond to the voltagelevels of the gate signal line 213, the reset signal line 214, and thephotosensor reference signal line 212 in FIG. 6C. A signal 604corresponds to the voltage level of the gate of the transistor 205 andshows the case where the illuminance of light which enters thephotodiode 204 is middle (hereinafter, middle illuminance). A signal 605shows the voltage level of the photosensor output signal line 216. Asignal 606 shows the voltage level of a node 611 in FIG. 6C.

The timing chart of FIG. 10 is described. In a period A, the voltagelevels of the signal 601 and the signal 603 are set to “H”, and thevoltage level of the signal 602 is set to “L2”. Then, in a period B,when the voltage level of the signal 602 is set to “H”, the voltagelevel of the signal 604 is reset, and the voltage levels of the signal605 and the signal 606 are increased. That is, the period A and theperiod B are a reset operation period. Next, in a period C, when thevoltage level of the signal 601 is set to “L2”, the voltage level of thesignal 606 is decreased. Then, in a period D, the voltage levels of thesignal 602 and the signal 603 are set to “L2”. That is, the period C andthe period D are an accumulation operation period. Next, in a period E,when the voltage levels of the signal 601 and the signal 603 are set to“H”, the voltage level of the signal 604 and the voltage level of thesignal 606 become the same voltage level, and the voltage level of thesignal 605 changes in accordance with an output signal of thephotosensor. That is, the period E is a selection operation period.Then, in a period F, the voltage levels of the signal 601 and the signal603 are set to “L2”, and the voltage level of the signal 605 is read.That is, the period F is a reading operation period. In such a manner,the operation of the photosensor 106 illustrated in FIG. 6C may beperformed.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 4

In this embodiment, a structure and manufacturing method of asemiconductor device including a photosensor will be described. FIG. 7is a cross-sectional view of a semiconductor device. Note that thefollowing semiconductor device can be applied to a display device.

In FIG. 7, a photodiode 1002, a transistor 1003, and a transistor 1004are provided over a substrate 1001 having an insulating surface. Thephotodiode 1002, the transistor 1003, and the transistor 1004 arerespectively cross-sectional views of the photodiode 204, the transistor205, and the transistor 206 in FIG. 2. Light 1202 emitted from an objectto be detected 1201, external light 1202 reflected by the object to bedetected 1201, or light 1202 emitted from the inside of the device andreflected by the object to be detected 1201 enters the photodiode 1002.An object to be detected may be provided on the substrate 1001 side.

The substrate 1001 can be an insulating substrate (e.g., a glasssubstrate or a plastic substrate), the insulating substrate on which aninsulating film (e.g., a silicon oxide film or a silicon nitride film)is formed, a semiconductor substrate (e.g., a silicon substrate) onwhich the insulating film is formed, or a metal substrate (e.g., analuminum substrate) on which the insulating film is formed.

The photodiode 1002 is a lateral-junction pin diode and includes asemiconductor film 1005. The semiconductor film 1005 includes a regionhaving p-type conductivity (a p-layer 1021), a region having i-typeconductivity (an i-layer 1022), and a region having n-type conductivity(an n-layer 1023). Note that the photodiode 1002 may be a pn diode.

The lateral junction pin or pn diode can be formed by adding a p-typeimpurity and an n-type impurity to predetermined regions of thesemiconductor film 1005.

In the photodiode 1002, a single crystal semiconductor (e.g., singlecrystal silicon) with few crystal defects is preferably used for thesemiconductor film 1005 so as to improve the proportion of an electricsignal generated from incident light (the quantum efficiency).

The transistor 1003 is a top-gate thin film transistor and includes asemiconductor film 1006, a gate insulating film 1007, and a gateelectrode 1008.

The transistor 1003 has a function of converting a charge supplied fromthe photodiode 1002 into an output signal. Therefore, a single crystalsemiconductor (e.g., single crystal silicon) is preferably used for thesemiconductor film 1006 to obtain a transistor with high mobility.

An example of forming the semiconductor film 1005 and the semiconductorfilm 1006 with the use of a single crystal semiconductor will bedescribed. A damaged region is formed at a desired depth of a singlecrystal semiconductor substrate (e.g., a single crystal siliconsubstrate) by ion irradiation or the like. The single crystalsemiconductor substrate and the substrate 1001 are bonded to each otherwith an insulating film interposed therebetween; then, the singlecrystal semiconductor substrate is split along the damaged region,whereby a semiconductor film is formed over the substrate 1001. Thesemiconductor film is processed (patterned) into a desired shape byetching or the like, so that the semiconductor film 1005 and thesemiconductor film 1006 are formed. Since the semiconductor film 1005and the semiconductor film 1006 can be formed in the same process, costreduction can be realized. In this manner, the photodiode 1002 and thetransistor 1003 can be formed on the same surface.

Note that an amorphous semiconductor, a microcrystal semiconductor, apolycrystalline semiconductor, an oxide semiconductor, or the like canalso be used for the semiconductor film 1005 and the semiconductor film1006. In particular, a single crystal semiconductor is preferably usedto obtain a transistor with high mobility. As the semiconductormaterial, it is preferable to use silicon semiconductor such as siliconor silicon germanium, the crystallinity of which can be easilyincreased.

Here, the semiconductor film 1005 is preferably made thick in order toimprove the quantum efficiency of the photodiode 1002. Further, thesemiconductor film 1006 is preferably made thin in order to improve theelectrical characteristics such as the S value of the transistor 1003.In that case, the semiconductor film 1005 is only required to be madethicker than the semiconductor film 1006.

A crystal semiconductor is also preferably used for the transistor 208in FIG. 2 so as to obtain a transistor with high mobility. By using thesame semiconductor material as the transistor 1003, the transistor 208can be formed in the same process as the transistor 1003, resulting incost reduction.

Note that the gate insulating film 1007 is formed as a single layer orstacked layers using a silicon oxide film, a silicon nitride film, orthe like. The gate insulating film 1007 may be formed by plasma CVD orsputtering.

Note that the gate electrode 1008 is formed as a single layer or stackedlayers using a metal material such as molybdenum, titanium, chromium,tantalum, tungsten, aluminum, copper, neodymium, or scandium, or analloy material including any of these materials as a main component. Thegate electrode 1008 may be formed by sputtering or vacuum evaporation.

The photodiode 1002 can have a stacked structure of a p-layer, ani-layer, and an n-layer instead of the lateral-junction structure. Thetransistor 1003 can be a bottom-gate transistor, and can have achannel-stop structure or a channel-etched structure.

Note that as illustrated in FIG. 9, a light-blocking film 1301 may beprovided under the photodiode 1002, so that light other than light whichshould be detected can be blocked. A light-blocking film may be providedover the photodiode 1002. In that case, a light-blocking film can beprovided, for example, over a substrate 1302 opposite to the substrate1001 provided with the photodiode 1002

The transistor 1004 is a bottom-gate inverted-staggered thin filmtransistor and includes a gate electrode 1010, a gate insulating film1011, a semiconductor film 1012, an electrode 1013, and an electrode1014. An insulating film 1015 is provided over the transistor 1004. Notethat the transistor 1004 may be a top-gate transistor.

A feature of the structure is that the transistor 1004 is formed overthe photodiode 1002 and the transistor 1003 with an insulating film 1009interposed therebetween. When the transistor 1004 and the photodiode1002 are formed on different layers in this manner, the area of thephotodiode 1002 can be increased to increase the amount of lightreceived by the photodiode 1002.

Furthermore, part or the whole of the transistor 1004 is preferablyformed to overlap with either the n-layer 1023 or the p-layer 1021 ofthe photodiode 1002. This is because the area of the photodiode 1002 canbe increased and the overlapping area of the transistor 1004 and thei-layer 1022 can be made as small as possible so that light can bereceived efficiently. Also in the case of a pn diode, a smalleroverlapping area of the transistor 1004 and a pn junction enablesefficient light reception.

The function of the transistor 1004 is to accumulate an output signal ofthe photodiode 1002 as a charge in the gate of the transistor 1003 andretain the charge. Therefore, an oxide semiconductor is preferably usedfor the semiconductor film 1012 so that the transistor has an extremelylow off-current.

It is also preferable that the transistor 207 in FIG. 2 use an oxidesemiconductor to have an extremely low off-current. By using the samesemiconductor material as the transistor 1004, the transistor 207 can beformed in the same process as the transistor 1004, resulting in costreduction. Note that for each of the above semiconductor elements, athin film semiconductor or a bulk semiconductor may be used.

An example of forming the semiconductor film 1012 using an oxidesemiconductor will be shown below.

One of the factors that increase the off-current of a transistor is animpurity such as hydrogen (e.g., hydrogen, water, or a hydroxyl group)contained in an oxide semiconductor. Hydrogen or the like might be acarrier supplier (a donor) in an oxide semiconductor, which causeselectric current even in the off state. That is, an oxide semiconductorcontaining a large amount of hydrogen or the like becomes an n-typeoxide semiconductor.

Thus, in the manufacturing method shown below, the amount of hydrogen inan oxide semiconductor is reduced as much as possible and theconcentration of oxygen which is a constituent element is increased,whereby the oxide semiconductor is highly purified. The highly-purifiedoxide semiconductor is an intrinsic or substantially intrinsicsemiconductor, resulting in a reduction in off-current.

First, an oxide semiconductor film is formed over the insulating film1009 by sputtering.

As a target used for forming the oxide semiconductor film, a target of ametal oxide containing zinc oxide as a main component can be used. Forexample, it is possible to use a target with a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:1, that is, In:Ga:Zn=1:1:0.5. It is also possible touse a target with a composition ratio of In:Ga:Zn=1:1:1 or a compositionratio of In:Ga:Zn=1:1:2. Further, a target which includes SiO₂ at 2 wt %to 10 wt % inclusive can be used.

Note that the oxide semiconductor film may be formed in a rare gas(typically, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere of a rare gas and oxygen. Here, a sputtering gas used forforming the oxide semiconductor film is a high-purity gas in whichimpurities such as hydrogen, water, hydroxyl groups, or hydride arereduced to such a level that the concentration thereof can be expressedby ppm, preferably ppb.

The oxide semiconductor film is formed by introducing a sputtering gasfrom which hydrogen and moisture are removed while removing moistureremaining in a treatment chamber. In order to remove moisture remainingin the treatment chamber, an entrapment vacuum pump is preferably used.For example, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used.

The thickness of the oxide semiconductor film may be 2 nm to 200 nminclusive, preferably 5 nm to 30 nm inclusive. Then, the oxidesemiconductor film is processed (patterned) into a desired shape byetching or the like, whereby the semiconductor film 1012 is formed.

Although an In—Ga—Zn—O is used for the oxide semiconductor film in theabove example, the following oxide semiconductors can also be used:In—Sn—Ga—Zn—O, In—Sn—Zn—O, In—Al—Zn—O, Sn—Ga—Zn—O, Al—Ga—Zn—O,Sn—Al—Zn—O, In—Zn—O, Sn—Zn—O, Al—Zn—O, Zn—Mg—O, Sn—Mg—O, In—Mg—O, In—O,Sn—O, Zn—O, and the like. The oxide semiconductor film may contain Si.Further, the oxide semiconductor film may be amorphous or crystalline.Further, the oxide semiconductor film may be non-single-crystal orsingle crystal.

As the oxide semiconductor film, a thin film represented byInMO₃(ZnO)_(m) (m>0) can also be used. Here, M denotes one or more ofmetal elements selected from Ga, Al, Mn, and Co. For example, M can beGa, Ga and Al, Ga and Mn, or Ga and Co.

Next, first heat treatment is performed on the oxide semiconductor film(the semiconductor film 1012). The temperature of the first heattreatment is higher than or equal to 400° C. and lower than or equal to750° C., preferably higher than or equal to 400° C. and lower than thestrain point of the substrate.

Through the first heat treatment, hydrogen, water, hydroxyl groups, andthe like can be removed from the oxide semiconductor film (thesemiconductor film 1012) (dehydrogenation treatment). Thedehydrogenation treatment through the first heat treatment issignificantly effective because such impurities become donors in theoxide semiconductor film and increase the off-current of the transistor.

Note that the first heat treatment can be performed with an electricfurnace. Alternatively, heat conduction or heat radiation from a heatingelement such as a resistance heating element may be used for the firstheat treatment. In that case, a rapid thermal anneal (RTA) apparatussuch as a gas rapid thermal anneal (GRTA) apparatus or a lamp rapidthermal anneal (LRTA) apparatus can be used.

An LRTA apparatus is an apparatus for heating an object to be processedby radiation of light (an electromagnetic wave) emitted from a lamp suchas a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high-pressure sodium lamp, or a high-pressure mercury lamp.

A GRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the gas, an inert gas (typically, a rare gassuch as argon) or a nitrogen gas can be used. The use of the GRTAapparatus is particularly effective because high-temperature heattreatment in a short time is enabled.

The first heat treatment may be performed before the patterning of theoxide semiconductor film, after the formation of the electrode 1013 andthe electrode 1014, or after the formation of the insulating film 1015.However, the first heat treatment is preferably performed before theformation of the electrode 1013 and the electrode 1014 so that theelectrodes are not damaged by the first heat treatment.

During the first heat treatment, oxygen deficiencies might be generatedin the oxide semiconductor. Therefore, after the first heat treatment,oxygen is preferably introduced to the oxide semiconductor (treatmentfor supplying oxygen) so that the concentration of oxygen which is aconstituent element is increased, whereby the oxide semiconductor ishighly purified.

Specifically, as the treatment for supplying oxygen, the first heattreatment is followed by second heat treatment in an oxygen atmosphereor an atmosphere containing nitrogen and/or oxygen (for example, thevolume ratio of nitrogen to oxygen is 4 to 1), for example.Alternatively, plasma treatment may be performed in an oxygenatmosphere, so that the oxygen concentration in the oxide semiconductorfilm can be increased and the oxide semiconductor film can be highlypurified. The temperature of the second heat treatment is higher than orequal to 200° C. and lower than or equal to 400° C., preferably higherthan or equal to 250° C. and lower than or equal to 350° C.

As another example of the treatment for supplying oxygen, an oxideinsulating film (the insulating film 1015) is formed on and in contactwith the semiconductor film 1012, and then third heat treatment isperformed. Oxygen in the insulating film 1015 moves to the semiconductorfilm 1012 to increase the oxygen concentration in the oxidesemiconductor, whereby the oxide semiconductor film can be highlypurified. The temperature of the third heat treatment is higher than orequal to 200° C. and lower than or equal to 400° C., preferably higherthan or equal to 250° C. and lower than or equal to 350° C. Note thatalso in the case of a top-gate transistor, the oxide semiconductor canbe highly purified in such a manner that a gate insulating film on andin contact with the semiconductor film 1012 is formed of a silicon oxidefilm or the like and similar heat treatment is performed.

As described above, the oxide semiconductor film can be highly purifiedthrough the treatment for supplying oxygen such as the second heattreatment or the third heat treatment after the dehydrogenationtreatment by the first heat treatment. When being highly purified, theoxide semiconductor can be made intrinsic or substantially intrinsic,resulting in a reduction in the off-current of the transistor 1004.

Note that the insulating film 1009 is a single layer or stacked layersusing a silicon oxide film, a silicon nitride film, or the like, and isformed over the photodiode 1002 and the transistor 1003. The insulatingfilm 1009 may be formed by plasma CVD or sputtering. The insulating film1009 may also be formed of a resin film such as a polyimide film bycoating or the like.

The gate electrode 1010, which is formed over the insulating film 1009,is formed as a single layer or stacked layers using a metal materialsuch as molybdenum, titanium, chromium, tantalum, tungsten, aluminum,copper, neodymium, or scandium, or an alloy material including any ofthese materials as a main component. The gate electrode 1010 may beformed by sputtering or vacuum evaporation.

The gate insulating film 1011 is formed as a single layer or stackedlayers using a silicon oxide film, a silicon nitride film, or the like.The gate insulating film 1011 may be formed by plasma CVD or sputtering.

The electrode 1013 and the electrode 1014, which are formed over thegate insulating film 1011 and the semiconductor film 1012, each are asingle layer or stacked layers using a metal such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, or yttrium, analloy material including any of these materials as a main component, ora metal oxide having conductivity such as indium oxide. The electrode1013 and the electrode 1014 may be formed by sputtering or vacuumevaporation. Here, it is preferable that the electrode 1013 be connectedto the n-layer 1023 of the photodiode 1002 through a contact hole formedin the gate insulating film 1007, the insulating film 1009, and the gateinsulating film 1011.

The highly-purified oxide semiconductor and a transistor using the samewill be described in detail below.

As an example of the highly-purified oxide semiconductor, there is anoxide semiconductor whose carrier concentration is lower than1×10¹⁴/cm³, preferably lower than 1×10¹²/cm³, and more preferably lowerthan 1×10¹¹/cm³ or lower than 6.0×10¹⁰/cm³.

A transistor using a highly-purified oxide semiconductor ischaracterized in that the off-current is much lower than that of atransistor including a semiconductor containing silicon, for example.

The following shows the result of measuring the off-currentcharacteristics of a transistor with an evaluation element (alsoreferred to as TEG: Test Element Group). Note that the description ismade here on an n-channel transistor.

In the TEG, a transistor with L/W=3 μm/10000 μm, which includes 200transistors with L/W=3 μm/50 μm (thickness d: 30 nm) connected inparallel, is provided. FIG. 8 illustrates the initial characteristics ofthe transistor. Here, VG is in the range of −20 V to +5 V inclusive. Inorder to measure the initial characteristics of the transistor, thecharacteristics of changes in the source-drain current (hereinafter,referred to as a drain current or ID), i.e., VG-ID characteristics, weremeasured under the conditions where the substrate temperature was set toroom temperature, the voltage between the source and the drain(hereinafter, referred to as a drain voltage or VD) was set to 1 V or 10V, and the voltage between the source and the gate (hereinafter,referred to as a gate voltage or VG) was changed from −20 V to +20 V.

As illustrated in FIG. 8, the transistor with a channel width W of 10000μm has an off-current of 1×10⁻¹³ A or less at VD of 1 V and 10 V, whichis less than or equal to the resolution (100 fA) of a measurement device(a semiconductor parameter analyzer, Agilent 4156C manufactured byAgilent Technologies Inc.). The off-current per micrometer of thechannel width corresponds to 10 aA/μm.

Note that in this specification, the off-current (also referred to asleakage current) means a current flowing between a source and a drain ofan n-channel transistor when a predetermined gate voltage in the rangeof −20 V to −5 V inclusive is applied at room temperature in the casewhere the n-channel transistor has a positive threshold voltage V_(th).Note that the room temperature is 15° C. to 25° C. inclusive. Atransistor including the oxide semiconductor that is disclosed in thisspecification has a current per unit channel width (W) of 100 aA/μm orless, preferably 1 aA/μm or less, and more preferably 10 zA/μm or lessat room temperature.

Moreover, the transistor including a high-purity oxide semiconductor hasfavorable temperature characteristics. Typically, in the temperaturerange of −25° C. to 150° C. inclusive, the current-voltagecharacteristics of the transistor, such as an on-current, anoff-current, field-effect mobility, an S value, and a threshold voltage,hardly change and deteriorate due to temperature.

This embodiment can be combined with any of the other embodiments asappropriate.

This application is based on Japanese Patent Application serial no.2010-050776 filed with Japan Patent Office on Mar. 8, 2010, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a photosensorcomprising: a first transistor; a second transistor; a third transistor;and a fourth transistor, wherein a first terminal of the firsttransistor is electrically connected to a first terminal of the secondtransistor and a gate of the third transistor, wherein a first terminalof the third transistor is electrically connected to a first terminal ofthe fourth transistor, and wherein a voltage level of a gate of thefirst transistor is lower than a voltage level of the first terminal ofthe first transistor and a voltage level of a second terminal of thefirst transistor, and a voltage level of a gate of the second transistoris lower than a voltage level of the first terminal of the secondtransistor and a voltage level of a second terminal of the secondtransistor when a charge corresponding to an amount of incident light tothe photosensor and accumulated to the gate of the third transistor isretained by turning off the first transistor and the second transistor.3. The semiconductor device according to claim 2, wherein the firsttransistor comprises a channel formation region comprising an oxidesemiconductor.
 4. The semiconductor device according to claims 2,wherein the second transistor comprises a channel formation regioncomprising an oxide semiconductor.
 5. The semiconductor device accordingto claim 2, wherein the first transistor comprises a channel formationregion comprising an oxide semiconductor, and wherein the secondtransistor comprises a channel formation region comprising an oxidesemiconductor.
 6. The semiconductor device according to claim 2, whereinthe charge is discharged from the gate of the third transistor byturning on the second transistor.
 7. The semiconductor device accordingto claim 2, wherein the charge is discharged from the gate of the thirdtransistor by turning on the first transistor and the second transistor.8. A semiconductor device comprising: a photosensor comprising: a firsttransistor; a second transistor; and a third transistor, wherein a firstterminal of the first transistor is electrically connected to a firstterminal of the second transistor and a gate of the third transistor,and wherein a voltage level of a gate of the first transistor is lowerthan a voltage level of the first terminal of the first transistor and avoltage level of a second terminal of the first transistor, and avoltage level of a gate of the second transistor is lower than a voltagelevel of the first terminal of the second transistor and a voltage levelof a second terminal of the second transistor when a chargecorresponding to an amount of incident light to the photosensor andaccumulated to the gate of the third transistor is retained by turningoff the first transistor and the second transistor.
 9. The semiconductordevice according to claim 8, wherein the first transistor comprises achannel formation region comprising an oxide semiconductor.
 10. Thesemiconductor device according to claim 8, wherein the second transistorcomprises a channel formation region comprising an oxide semiconductor.11. The semiconductor device according to claim 8, wherein the firsttransistor comprises a channel formation region comprising an oxidesemiconductor, and wherein the second transistor comprises a channelformation region comprising an oxide semiconductor.
 12. Thesemiconductor device according to claim 8, wherein the charge isdischarged from the gate of the third transistor by turning on thesecond transistor.
 13. The semiconductor device according to claim 8,wherein the charge is discharged from the gate of the third transistorby turning on the first transistor and the second transistor.
 14. Asemiconductor device comprising: a photosensor comprising: a firsttransistor; and a second transistor; wherein a first terminal of thefirst transistor is electrically connected to a gate of the secondtransistor, wherein a voltage level of a gate of the first transistor islower than a voltage level of the first terminal of the first transistorand a voltage level of a second terminal of the first transistor when acharge corresponding to an amount of incident light to the photosensorand accumulated to the gate of the second transistor is retained byturning off the first transistor.
 15. The semiconductor device accordingto claim 14, wherein the first transistor comprises a channel formationregion comprising an oxide semiconductor.
 16. The semiconductor deviceaccording to claim 14, wherein the second transistor comprises a channelformation region comprising an oxide semiconductor.
 17. Thesemiconductor device according to claim 14, wherein the first transistorcomprises a channel formation region comprising an oxide semiconductor,and wherein the second transistor comprises a channel formation regioncomprising an oxide semiconductor.
 18. The semiconductor deviceaccording to claim 14, wherein the charge is discharged from the gate ofthe second transistor by turning on the first transistor.